This invention relates generally to the field of pressure transducers. More particularly, it relates to a sensor for such transducers which employs a pressure-responsive diaphragm carrying solid-state, piezoresistive elements.
One type of pressure transducer which has generated much interest in recent years is the type which employs a pressure-responsive diaphragm of dielectric material on which are epitaxially grown semiconductor piezoresistive strain gauges. The strain gauges are formed in an appropriate circuit, such as a bridge circuit, so that a deflection of the diaphragm in response to a pressure produces a pressure-indicative voltage signal. Typical materials for the diaphragm are monocrystalline aluminum oxide (i.e., sapphire), beryllium oxide, and spinel. Silicon is the predominant choice for the strain gauge material. Examples of this type of transducer can be found in U.S. Pat. No. 3,916,365 to Giachino and U.S. Pat. No. 4,127,840 to House.
In a typical application, a diaphragm of the type described above, is sandwiched between upper and lower housing elements. More specifically, the diaphragm is bonded, at the periphery of its lower side, to a lower housing element, and at the periphery of its upper side to an upper housing element, with the strain gauges being located in a central "active area", usually on one side or the other of the diaphragm. It is desirable, especially where the transducer is to be exposed to widely-varying temperatures, to form the housing elements of a material having a thermal coefficient of expansion which is closely matched to that of the diaphragm. Without such a match, the diaphragm would undergo stresses as the temperature varied, thereby yielding erroneous readings due to, for example, null point shifting. Thus, in a transducer having a sapphire diaphragm it is highly advantageous to form the housing elements of amorphous aluminum oxide.
For the same reason, it is desirable to provide a bonding agent between the diaphragm and the housing elements which has a coefficient of thermal expansion approximately equal to those of the diaphragm and of the housing elements. Typically, the bonding agent is a glass provided as a frit in a raw form. It is within the expertise of those of typical skill in the pertinent arts to formulate a glass of suitable composition having a coefficient of thermal expansion approximately equal to that of, e.g., aluminum oxide, upon fusing at high temperature.
One problem experienced with the construction described above is that the fused glass bonding material, although having high compression strength, has relatively low tensile strength. The low tensile strength of this glass material can result in the fracturing of one or both of the bonding layers (between the diaphragm and the upper housing element, and between the diaphragm and the lower housing element) as the diaphragm is loaded by pressures approaching or exceeding its overpressure limit. This result is due to the fact that, at the inside edges of these bonding layers, the tensile loading exceeds the compressive loading for any significant axial deflection of the diaphragm's central active area.
The prior art has approached this problem by seeking structurally to isolate the bonding layers from tensile stresses, thereby subjecting these layers, ideally, only to compressive loading. This approach has resulted in complex designs that are relatively costly and difficult to manufacture.
From the foregoing, it can be seen that a pressure sensor design which minimizes the problem of bonding layer fracture from tensile loading, without the need for a complex mechanical structure, would fill a long-felt, yet unsatisfied need in the pertinent industry.